Electronically controlled fuel injection systems must be capable of high speed operation and must have consistently reproducible stroke characteristics. High speed solenoid operation is therefore imperative as slow acting solenoids result in erroneous quantities of fuel being delivered to each cylinder at an inappropriate timing advance which can adversely affect the performance of the engine. A fuel injection solenoid control system can provide advantageous control of engine operation over the entire range of engine speeds by delivering a regulated voltage for a variable duration of time. This results in the solenoid opening the fuel injector for a substantially standard time duration to deliver a substantially standard pre-selected quantity of fuel to each individual cylinder. Typically, the rise time of current flow through the solenoid is a function of the voltage applied. The reproducibility of the stroke characteristics versus the control signal applied to the solenoid improves with higher voltages applied to the solenoid. However, higher voltages typically require higher voltage supplies that add to the expense of the overall driver circuit.
In a typical fuel injection system for a multi-cylinder engine, a fuel injection solenoid is provided for each engine cylinder and power to each solenoid must be supplied and removed for each compression stroke. Typically, the energy stored in the solenoid during energization is transformed into heat by a diode and resistor combination placed in the flyback current path of each solenoid when power is removed from the solenoid. The magnitude of the energy disposed of in this manner is significant and directly results in an increase to the cost of the system.
U.S. Pat. No. 5,717,562 which issued to the present assignee addresses some of the drawbacks associated with the prior art solenoid driver circuits and discloses an energy saving solenoid driver circuit which recovers power normally dissipated by the flyback current path in a conventional solenoid driver. More particularly, the solenoid driver circuit disclosed in U.S. Pat. No. 5,717,562 provides the advantages of a high voltage solenoid driver while eliminating many of the circuit components of the high voltage power supply traditionally associated with such high voltage solenoid drivers, and such driver circuit primarily recaptures solenoid coil energy (back EMF) when power is disconnected from the solenoid coil, that is, when fuel injection for that particular stroke is complete. Also, the high voltage capacitor associated with the driver circuit disclosed in U.S. Pat. No. 5,717,562 is only charged from the back EMF associated with the particular injector solenoid coil located in that particular driver circuit, and such back EMF is not utilized to recharge any high voltage capacitors associated with the other solenoid injector driver circuits in a typical fuel injection system.
It is therefore desirable to provide an energy recovery circuit configuration wherein the high voltage capacitors associated with a plurality of solenoid injector driver circuits can be recharged simultaneously from the back EMF associated with any one or a plurality of the injector solenoid coils associated with such driver circuits, and that such recharging of at least some of the high voltage capacitors can take place even while the injector solenoid coils for some of the driver circuits are being powered by their capacitors for fuel injection to particular cylinders.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.